1. Field of the Invention
This invention relates to acousto-optical signal processing and more particularly to acousto-optical means for modulating electrical signals.
2. Description of the Prior Art
Simulation of the transmitted radar pulse and a target return signal is most often provided by generating a pulse modulated carrier in a delay line in response to a simulated radar transmission pulse. A delayed pulse provided by the delay line simulates the target return signal. Therefore, simulation of a target signal representative of a single point target at an arbitrary distance (range) from a radar apparatus is minimal in its complexity. Target signals from point targets, however, are atypical of signals received from actual radar targets. Typical radar targets may include combinations of chaff, extended targets and multiple targets.
A target signal received from an extended target may be phase modulated and of longer duration than the transmitted pulse. When the target is large and there is a variation of the range of different parts of the target, there may be a difference of time between the arrival at the radar antenna of portions of the target signal reflected from the different parts of the target, thereby causing a frequency modulation of the carrier and an elongation of the target signal.
Chaff and multiple targets provide a multiplicity of target signals in response to a single transmitted pulse. A realistic simulation of multiple target signals requires a simulated target signal corresponding to each target, the range of each target being associated with the time interval between the generation of a simulated transmitted pulse and the generation of a simulated target signal; the phase modulation and elongation of each target signal caused by the size of the corresponding target must also be simulated.
Movement of a target, towards or away from the radar antenna, frequency modulates the carrier of the target signal thereby causing a Doppler frequency shift of the carrier frequency; the Doppler shift must be provided in a realistic simulation which includes the movement of a target.
The inventions of Brienza disclosed in U.S. Pat. Nos. 3,463,573; 3,539,245 and 3,644,742 and DeMaria et al in U.S. Pat. Nos. 3,544,806; 3,517,332; 3,485,559 and 3,566,303, all assigned to the same assignee, describe how an electrical input signal may be processed by acousto-optical interaction whereby a processed electrical output signal is provided. These patents do not disclose a means for phase modulating the electrical signal which, for reasons explained hereinbefore, is required in applications such as the realistic simulation of coherent radar return signals.
A simulation of radar signals typically received by a radar receiver has not been known in the prior art because apparatus of great complexity has heretofore been required for a realistic simulation. One aspect of the present invention relates to the phase modulation of electrical signals whereby radar signals may be readily simulated.